1. Field of the Invention
This invention relates generally to apparatus for monitoring the heart rate of a fetus and more specifically to such apparatus adapted to transmit signals through hospital power lines or by direct wiring to a remote monitoring station.
2. Description of the Prior Art
In recent years medicine has made significant advances in many areas related to the care of mothers and their babies but the number of children born with brain damage has not decreased significantly. It is believed that in certain of these babies, the brain damage has occurred during labor or at birth due to a reduction in the supply of oxygen.
It has been found that by monitoring the fetus, doctors can be provided with a continuous and highly accurate report on the most widely used indicator of fetal condition, the fetal heart rate. By monitoring the fetal heart rate, doctors have been able to decrease the need for emergency Cesarean section. This decrease has been made possible by conservative measures such as changing the maternal position to relieve pressure on the umbilical cord.
Various apparatus have been used for monitoring the fetal heart rate. For example stethoscopes have been used to listen to the fetal heart beat. This particular means for monitoring the heart rate has been unsatisfactory during the periods of labor contractions due to the high degree of extraneous or artifact noise. Since fetal distress is apt to occur during labor contractions, the stethoscopic means for monitoring the heart rate have not provided a satisfactory indication of fetal condition.
The heart rate of the fetus has also been monitored using various electronic apparatus to determine certain characteristics of the heart beat. This is usally accomplished using two electrodes one of which is attached to the head of the fetus. Although this technique has provided reasonably accurate data, there are obvious disadvantages. For example, the electrode can not be attached to the head of the fetus until the cervix has opened sufficiently and the amniotic sack has been ruptured. For obvious reasons, this technique has not been used for monitoring the fetus in the early stages of labor. In addition the electrode has been applied either blindly or by using a special light source which requires special training. Furthermore, the presence of a doctor has been required to supervise the electrode attachment.
Ultrasonic transducers have also been used for monitoring the fetal heart rate. These transducers have transmitted an ultrasonic signal which has been reflected by the motion of tissue, such as the heart, to produce a Doppler effect and a corresponding electrical signal in the transducer. Since any movement of tissue within the path of the transmitted ultrasonic signal produces characteristics in the corresponding electrical signal, the particular characteristics associated with the heart beat are disguised in a high degree of extraneous or artifact noise.
The resulting electrical signal is of relatively low power and the characteristics of particular interest are of relatively low frequency. As a result, it has been particularly difficult to isolate these signals from the spurious low frequency signals in the atmosphere such as those transmitted by frequency modulated radio stations. Monitors of the prior art receiving these crude signals have been unable to satisfactorily separate the characteristics relating to the heart rate from the characteristics relating to the artifact noise and radio interference. As a consequence other types of sensors have typically been used in conjunction with the ultrasonic sensor to monitor the heart rate.
It has been desirable to monitor the fetal heart rate not only in the labor room and the delivery room but also at a remote location such as the nurses station. In some systems of the prior art, monitors have been provided in the labor room and hard wires have been run in the hospital walls between the labor room and the nurses station for each of the heart rates monitored. At the nurses station a single, self-contained unit has been used for each of the heart rates monitored.
The ultrasonic sensors, which have typically been attached to the abdomen of the mother with adhesive tape, have had a resonant frequency which has been matched by signal characterizing electronics in the receivers of the monitors. This matching has been desirable to maximize the power of the relatively crude signal received from the sensors. To facilitate the matching, each of the ultrasonic sensors has been designed for operation with a particular one of the monitors. As a consequence, when a mother has been removed from the labor room to the delivery room, the sensor associated with the labor room monitor has been removed and the sensor associated with the delivery room monitor has been attached. This procedure has been time consuming and the removal of the adhesive tape with the sensor has been particularly annoying to the mother.
At the nurses station, the monitors have included a meter providing an instantaneous indication of heart rate. Alarm means have provided an audible indication when the heart rate is above or below a preferred range of heart rates. When a nurse has been at the station and the alarm has sounded, it has been possible to read the meter to determine if the heart rate is high or low. In some cases, the nurse has been away from the station when the alarm has sounded, and the meter has returned to the preferred range prior to her return to the station. Although in some systems the alarm has remained active, there has been no indication as to whether the rate was higher or lower than the preferred range of heart rate.
When the ultrasonic sensor has become detached from the mother, the heart rate indication of substantially zero has provided the alarm indication. This detachment of the sensor has occurred quite frequently and the alarm has caused a high degree of concern when it was not particularly warranted. There has been no way to distinguish a heart rate of low level from a heart rate of substantial zero level.
When the sensor has first been attached, the heart rate has gradually built up to indicate a particular value. Each time the sensor has become disconnected, this undesirable gradual buildup has followed the reattachment. There has been no means for storing the particular value of the heart rate to avoid the gradual buildup of the rate signal following the reconnection of the sensor. In some systems, the interpulse period has been used to determine the heart rate. Although without averaging, this approach could provide an instantaneous indication of heart rate, the display would be very jumpy.